Fuel cells have been proposed as a power source for electric vehicles and other applications. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. A common technique for storing large quantities of hydrogen is to cool and compress hydrogen via liquefaction techniques and store the liquid phase hydrogen in a cryogenic storage tank. Hydrogen gas liquefies at −253° C. and ambient pressure, and can be stored at about 70 g/L in the liquid phase. The amount of energy required to compress hydrogen gas into a liquid is very high, and currently may be as much as up to 40% of the energy obtained from using the gas as a fuel. Thus, it is advantageous to keep the liquid phase hydrogen as insulated as possible from the surrounding ambient temperature.
Any transfer of heat to the innermost portion of the cryogenic storage tank affects the natural evaporation rate of the cryogenic vessel. The more heat that is transferred, the faster the rate of boil-off of the liquid hydrogen, or the higher the natural evaporation rate. In order to maintain the hydrogen in a liquid state, thereby minimizing excess vaporization and the need to vent the tank in order to release excess pressure, heat transfer from the ambient environment to the cryogenic tank must be kept to a minimum.
Conventional cryogenic tanks, such as the tanks used to supply hydrogen gas to a fuel cell, are commonly made of aluminum or stainless-steel alloys. The storage tanks generally consist of an inner storage vessel encapsulated with an outer vessel, or shell. The vessels are commonly separated from one another with metal supports and the space between the inner vessel and the shell is well insulated and under a vacuum. Multi-layer insulation has been used for quite some time to insulate cryogenic tanks. For constant surface emissivities in a vacuum of 10−5 Torr or lower, heat transfer usually decreases with 1/N, where N is the number of reflective insulation layers disposed between the vessel's warm and cold surfaces.
One common type of vacuum-insulation system includes the use of “super-insulation” or “SI”. SI systems generally include multiple layers of metallized film along with a distribution of protrusions, or spacer materials, provided between the layers to prevent face-to-face contact when the films are wrapped around the inner tank and subsequently around one another. As one of the primary sources of heat transfer, the spacers bridge the insulation that is present, and allow heat from the ambient environment to penetrate into the inner vessel, leading to detrimental effects on the overall thermal insulation. Accordingly, there is a need for an improved cryogenic liquid storage tank, and particularly, one that minimizes heat convection between the insulation and inner and outer tanks.